Wavelength-division multiplexing is a technology which multiplexes a number of optical carrier signals onto a single optical path, such as a single optical fiber, by using different wavelengths. The optical carrier signals having different wavelengths are commonly referred to as channels. This technique enables bidirectional communications over an optical path. Generally, in WDM transmission systems it is important that the spectrum of the transmission source at the transmitter side and filter properties of wavelength selective elements at the receiver side are adjusted to each other. In particular, neighboring channels have to be prevented from excessively interfering with each other by crosstalk.
Optical bandpass filters may prevent or minimize interferences by neighboring WDM channels. The characteristics of these filters with respect to the center frequency bandwidth and further parameters may be defined exactly and may be adapted to the spectrum to be transmitted. In this regard, reference is made to Idler, et al.: “WDM Field Trial over 764 km SSMF with 16×112 Gb/s NRZ-DQPSK co-propagating with 10.7 Gb/s NRZ”, ECOC 2010, 19-23 Sep. 2010, Torino, Italy, We.8.C.5. Furthermore, it is important to prevent a mismatch between the spectrum of the signal source and the filter characteristics due to environmental conditions, such as thermal influences. To this end, temperature stabilization or a wide filter passband, such as coarse wavelength division multiplexing (CWDM), have been proposed, see ITU-T G.694.2 Spectral grids for WDM applications: CWDM wavelength grid, 12/2003 and ITU-T G.671 Transmission characteristics of optical components and subsystems, 01/2009.
On the other hand, in the field of wired electrical communication systems, compensation by means of digital signal processing is commonly used and investigated intensively. Examples are a parallel data transmission of four data streams in gigabit ethernet systems, see IEEE Std. 802.3 Local and metropolitan area networks—Specific requirements Part 3: Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications, Clause 40, 2005, or the parallel data transmission of two data streams in power line communication (PLC) systems, see ITU-T, “G.9963: Unified high-speed wireline-based home networking transceivers—Multiple input/multiple output specification”, 12/2011.
In optical systems, crosstalk compensation for space division multiplexing (SDM) systems was tested, see S. Chandarasekhar et al., “WDM/SDM Transmission of 10×128—Gb/s PDM—QPSK over 2688-km 7—Core Fiber with a per-Fiber Net Aggregate Spectral Efficiency Distance Product of 40,320 km. b/s/Hz”, ECOC Post-deadline Papers, 2011 OSA]. Such compensation was suggested under the key word MIMO-SDM, see C. Koebele, M. Salsi, L. Milord, R. Ryf, C. Bolle, P. Sillard, S. Bigo, and G. Charlet, 840 km transmission of five mode division multiplexed data streams at 100 Gb/s with low MIMO-DSP complexity, in Proc. ECOC, Geneva, Switherland, 2011, Paper Th. 13.C3.
Taiji Sakamoto, et al.: “Differential Mode Delay Managed Transmission Line for WDM-MIMO System Using Multi-Step Index Fiber”, JLT, Vol. 30, NO. 7, Sep. 1, 2012, use the term WDM-MIMO (Wavelength-Division Multiplexing Multiple Input Multiple Output) in the context of a system consisting of fibers with positive and negative differential mode delays to compensate for the total differential mode delay.